1. Field of the Invention
This invention relates to an improved method for the separation of indole by distilling an indole-containing reaction fluid obtained by using N-(.beta.-hydroxyethyl)aniline as the starting material or by using aniline and ethylene glycol as the starting materials.
2. Description of the Prior Art
Indole is known as an important material for use in chemical industry and, especially in recent years, has become essential for the synthesis of perfumes and amino acids.
In the past, many attempts were made to synthesize indole. However, most of the processes so devised had the disadvantages of yielding considerable amounts of by-products, requiring expensive compounds as the starting materials, leading to indole through a lengthy chain of steps and/or involving troublesome operations. Meanwhile, two single-step processes for the preparation of indole from inexpensive compounds have recently been proposed, one using N-(.beta.-hydroxyethyl)aniline as the starting material and the other using aniline and ethylene glycol as the starting materials. The findings of the present inventor have revealed that, in these two processes, the presence of a large amount of aniline in the reaction system is required to obtain indole in good yield. Accordingly, in the preparation of indole by these processes, it is essential to separate and recover the large amount of aniline contained in the resulting reaction fluid. Moreover, the reaction fluid resulting from either of these processes also contains trace amounts of by-products. Thus, the indole obtained by the mere separation of aniline still contains impurities and cannot be directly used as a starting material for the synthesis of perfumes and amino acids.
Even if it is tried to separate indole by distilling such a reaction fluid, a part of the indole contained in the reaction fluid is deteriorated (i.e., converted into a tarry substance) in the course of the distillation to cause a considerable loss of indole. As a result, the amount of indole thus obtained is significantly smaller than that of indole initially present in the reaction fluid.